EFFECTS OF TOPOLOGICALLY CLOSE PACKED μ PHASE ON MICROSTRUCTURE AND PROPERTIES IN POWDER METALLURGY Ni-BASED SUPERALLOY WITH Hf

It is widely acknowledged that topologically close packed (TCP) phases are detrimental to comprehensive properties of superalloys, as TCP phases deplete strengthening elements from matrix and easily become crack initiations. In this work, the precipitation kinetics and morphology of topologically close packed mu phase in FGH4097 powder metallurgy (PM) superalloy with (0 similar to 0.89%)Hf and the effect of mu phase on the mechanical prop erties of FGH4097 PM superalloy billet with 0.30%Hf has been investigated. The results showed that mu phase precipitated obviously in the alloys with 0.30%Hf and 0.89%Hf after long-term ageing at 750-900 degrees C, the amount and size of mu phase increased as the ageing temperature, ageing time and Hf content increasing. mu phase mainly precipitated in grains with strip and flake shapes. After long-term ageing at 550-650 degrees C, no mu phase precipitated in FGH4097 PM superalloy billet with 0.30%Hf and the tensile properties and stress-rupture properties at high temperature were not decreased, which showed excellent microstructure stability. After long term ageing at 750, precipitated mu phase had little effect on tensile strength at high temperature, however, the tensile ductility increased and high temperature stress rupture life reduced, and the stress rupture ductility increased by about 20%. In this work, the precipitation behavior of mu phase, the redistribution of elements in gamma solid solution and the FGH4097 PM superalloy fracture morphology characteristics have been discussed in detail. The mechanism of the brittle and ductile dual effect of mu phase on the mechanical properties has been explained. The methods of controlling and avoiding excessive mu phase precipitation which leaded to performance deterioration have been proposed.